Upper Resp. Problems Keep Coming Back

Has anybody heard of this problem?
Starting around November, several people I work with have had cold-like symptoms. Sinus infection, sore throat, coughing, laryngitis. At least 4 of us have been given Z pack antibiodics by our physicians. The symptoms go away, then after a few weeks, it comes back again.
I had this happen 3 times since Christmas. I've been through the antibiodics and steroids twice. The symptoms went away all except a minor cough at night. I went to the doctor, told him I, now, felt fine. He gives me a clean bill of health. Then 2 days later, all the symptoms re-occur. Sinus infection, sore throat, cough. 2 other people at work have told me thay have had the same thing happen to them. We all agree that each time it occurs, the sympthoms are less severe than the previous time.
What is this? Is this some kind of infection that is occuring around the country? (We all work in a hospital maternity floor).